1. Field of the Invention
This invention relates to methods and indicators of authenticity. The invention is particularly useful with documents, tags, and labels.
2. Description of the Related Art
Indicators to ascertain the genuineness of documents, tags and labels have been proposed in several different forms.
U.S. Pat. No. 3,063,163 to Carmeli teaches a method of detecting counterfeit currency substrates by applying an iodine solution to the substrate to react with starch yielding a brown to black marking. Carmeli is a negative indicator in that genuine currency does not undergo a color change. Counterfeit currency on the other hand forms a bluish black mark with the iodine indicator, attributed to a difference in starch content. The marking is fairly permanent, however can be made to dissipate over a few days by inclusion of oxidizing agent.
Wood U.S. Pat. No. 4,037,007 teaches secure documents with color forming planchettes. A color forming reaction of azo compounds or lactone or leuco compounds with an organic acid is taught for forming a color change or change from colorless to colored form. Over time or repeated use, such planchettes are undesirable as unsightly producing color changed or discolored areas of relative permanence.
U.S. Pat. No. 5,130,290 to Tanimoto teaches a reversible coloring sheet based on coating a basic dye and color developer onto a sheet together with a desensitizer such as polyethylene glycol. The resulting coated substrate develops a color upon contact with water which can reverse upon removal of the water. Although functional with only application of water, this system would have the drawback of unsolicited color change due to inadvertent wetting, rain, high humidity and other events attributable to the ubiquitous presence of water in many areas of application or conditions of use.
EP 0 530 059 discloses a system based on a complex formed by potassium thiocyanate with transition metals such as iron or copper. After color formation, a second material, a reducing agent such as sodium sulfite or bisulfite is applied to obliterate the color formed.
To date, no authentication system based on a color change upon contact with an applicator material has been described or suggested, which comprises a momentarily visible color change that reverts to a colorless form upon removal of the applicator, and which is resistant to unintended coloration due to inadvertent wetting.
Such an invention would be commercially useful providing a system that is not prone to image inadvertently and solves the problem of unsightly discoloration of the tested surface of the substrate.
It is an object of the present invention to disclose a rapidly reversing authenticity indicator. It is a further object of the invention to disclose a positive indicator.
It is a further object of the present invention to disclose a combination of colorless or lightly chromogenic materials in combination with a selected set of developers that yield momentary indicators.
It is a further object of the invention to disclose reversible authenticity indicators that can be employed with a substrate without permanent marking or unsightly discoloration.
It is a further object of the invention to disclose a rapidly reversing authenticity indicator that can be employed with a substrate to preclude inadvertent discoloration and to minimize false positives.